A surface-emission laser diode (surface-emission semiconductor laser) is a laser diode that emits light in a vertical direction to the substrate and is used for civil purposes, such as optical source of optical telecommunication including optical interconnection, optical source of optical pickup devices, optical source of image forming apparatuses, and the like.    Patent Reference 1 Japanese Laid-Open Patent Application 2002-164621    Patent Reference 2 Japanese Laid-Open Patent Application 9-107153 official gazette    Patent Reference 3 Japanese Laid-Open Patent Application 2001-60739    Patent Reference 4 Japanese Laid Open Patent Application 2001-168461    Non-patent Reference 1 IEEE Photonics Technology Letters, Vol. 10, No. 12, pp. 1676-1678, 1998 (Tokyo Institute of Technology)    Non-patent Reference 2 IEEE Photonics Technology Letters, Vol. 12, No. 6, pp. 603-605, 2000 (Wisconsin Univ.)
A surface-emission laser diode for such applications is typically targeted to the specifications that the laser diode has a large gain for the active layer, low threshold value, high output, excellent reliability, and controlled polarization direction.
Particularly, surface-emission laser diodes have a tendency of providing small optical output as compared with edge-emission laser diodes in view of the small volume of the active layer. Thus, there are many cases in which demand of increased output is imposed to such surface-emission laser diodes. One approach of increasing the optical output is to suppress the temperature rise of the optical emission part.
First, explanation will be made for a typical device structure of surface-emission laser diode and description will be made further with regard to the measures taken against heat.
FIG. 1 is a diagram showing an example structure of general surface-emission laser diode. It should be noted that the example of FIG. 1 is a surface-emission laser diode having InGaAs/GaAs for the active layer and using the current confinement structure formed by selective oxidization of an AlAs layer.
The general surface-emission laser diode shown in FIG. 1 is produced in the following manner.
By an MOCVD process or MBE process, there is formed a stack of films by consecutively stacking, on an n-GaAs single crystal substrate 11, an n-AlGaAs/n-AlGaAs lower semiconductor multilayer reflector (DBR: distributed Bragg reflector) 12, a lower GaAs spacer layer 13, an AlGaAs/AlGaAs-MQW active layer 14, an upper AlGaAs spacer layer 15, an AlAs layer for selective oxidation 16, and p-AlGaAs/p-AlGaAs upper semiconductor multilayer reflector (DBR).
Next, the stacked film is processed to form a mesa structure by a dry etching process. With this process, it is commonly practiced to conduct the etching such that the etching surface reaches a region inside the lower semiconductor DBR 12.
Next, the AlAs layer for selective oxidation 16 having the sidewall surface exposed by the dry etching process is processed in water vapor, such that there occurs oxidation at the peripheral part thereof, to form an insulation layer of AlxOy in such a peripheral part. Thereby, there is formed a current confinement structure that restricts the path of the device drive current only to the AlAs region at the central part where the oxidation has not taken place.
Next, the peripheral part of the mesa structure is filled with an insulation film 18, and a p-side electrode 19 and an n-side electrode 20 are formed at predetermined locations. With this, fabrication of the surface-emission laser diode of FIG. 1 is completed.
Meanwhile, with such a surface-emission laser diode, it is preferable to reduce the height of the mesa structure and thereby to reduce the heat resistance to the substrate for the purpose of improving heat dissipation from the active layer 14. However, with the mesa formation process conducted by a dry etching process, fluctuation in the etching depth of ±10% or more is not unusual, and further in view of the possible fluctuation of etching depth inside the specimen, it is inevitable to set the height of the mesa structure to be larger than the height which is actually necessary.
Patent Reference 1 shows the construction that reduces heat resistance.
With the construction shown in Patent Reference 1, AlAs, having a much larger thermal conductivity as compared with AlGaAs, is used for the low refractive index layers in the majority part of the DBR located at the lower part of the lower semiconductor DBR. On the other hand, conventional AlGaAs is used for the low refractive index layer in the upper part of lower semiconductor DBR. The reason of this is to avoid the situation that the etching surface reaches inside the lower part AlAs-DBR and there takes place oxidation at the edge surface of the AlAs layer of lower part AlAs-DBR exposed at the mesa sidewall surface at the time of conducting the oxidation processing of the AlAs layer for selective oxidation later. When this occurs, the active part in the device is insulated or there occurs increase of device resistance. The reason that such a situation arises is that the oxidation rate of AlGaAs relies heavily upon the Al content and that there occurs increase in the oxidation rate with increase of the Al content. Thus, the oxidation rate becomes the largest with AlAs.
In order to avoid this problem, Patent Reference 1 forms the upper AlGaAs-DBR by using AlGaAs of small oxidation rate and controls the bottom surface of etching such that the bottom surface of etching is located in this upper AlGaAs-DBR. Thereby, exposure of the AlAs edge surface of the lower AlAs-DBR is avoided. With such an upper AlGaAs-DBR, it is preferable that the pair number is equal to or smaller than 4/7 of the entire pair number as set forth in Patent Reference 1. Particularly, it is desirable to set the pair number to 10 pairs or less.
However, with the approach of controlling the bottom surface of etching such that the bottom surface of etching is located in the upper AlGaAs-DBR of only about 10 pairs by way of controlling the etching time, there is caused remarkable decrease of yield, and there arises a problem in that large fluctuation occurs in the position of the bottom surface of etching in the upper AlGaAs-DBR.
Therefore, in order to achieve the object of providing measures against heat while maintaining high yield, control of the mesa formation process by dry etching process becomes important. For this, it is desirable to carry out monitoring of the etching process.
For the monitoring method of dry etching process, there is a known method of plasma atomic emission spectrometry. Further, there is a method that irradiates the surface of the specimen to be etched with light, wherein this method monitors the intensity of the reflection light and detects the etching depth from the change of the interference intensity of the film. From the viewpoint that there is no need of providing observation window and in view of the fact that the method is well established and that there are commercially available instruments, it is thought advantageous to use the plasma atomic emission spectrometry.
With the monitoring method that uses this plasma atomic emission spectrometry, the change of emission intensity corresponding to the semiconductor film composition is detected by monitoring the atomic emission intensity of Ga at 417 nm or the time change of the ratio of the atomic emission intensity of Ga at 417 nm to the atomic emission intensity of Al at 396 nm. In the case of the specimen formed primarily of repetition of the layers of two compositions as in the case of the layered structure of the surface-emission laser diode, this atomic emission intensity change draws an oscillatory waveform.
However, in the semiconductor film used for the surface-emission laser diode that oscillates at the laser oscillation wavelength of a GaAs active layer (about 850 nm), in particular, the change of Ga composition or Ga/Al composition ratio is small. Further, the film thickness of the DBR or cavity is small in correspondence to the wavelength. Thus, the amplitude of the oscillatory waveform of the plasma atomic emission spectrometry becomes small. Thereby, it is not easy to carry out the monitoring. Further, in the case the specimen to be etched has large size, there arises a problem in that monitoring becomes difficult because of the distribution of etching rate inside the specimen.
Meanwhile, it is known that excellent carrier confinement to the active layer is attained in the surface-emission laser diode of the 850 nm band and 980 nm band.
For example, GaAs is used for the quantum well active layer and AlGaAs is used for the barrier layer and the spacer layer (cladding layer) of a surface-emission laser diode of the 850 nm band. Further, with such a surface-emission laser diode of the 850 nm band, it is possible to use the current confinement structure that uses a high-performance AlGaAs-system reflector (DBR) and the current confinement structure that uses an Al oxide film.
Further, various proposals have been made with regard to polarization control of such a surface-emission laser diode. For example, there is proposed a method of providing anisotropy in the outer shape of the active layer as viewed from the optical emission direction. Particularly, Non-Patent Reference 1 shows that polarization control is possible by anisotropy of optical gain realized by using a (311)B substrate, in other words a so-called off-substrate that is inclined from (100) by 25° in the (111)B direction, such that the optical gain in the inclined direction is increased. Further, it is shown that similar effect is obtained also with a (311)A substrate.
However, with the technology of Non-Patent Reference 1, there is a drawback in that it is difficult to conduct crystal growth on the heavily inclined (311)B substrate as compared with the crystal growth on the (100) substrate and that crystal growth on the (311)A substrate is even more difficult.
Further, in any of these substrates in which the substrate is heavily inclined, the cost of the substrate is increased by twice or more and it is difficult to carry out cleaving process. Further, handling of the substrate is difficult.
Meanwhile, the surface-emission laser diode of the wavelength shorter than 850 nm is realized by increasing the bandgap of the quantum well active layer by adding thereto Al.
For example, there is a proposal of a surface-emission laser diode of the 780 nm band in which Al is added to the quantum well active layer by about 12% in terms of the compositional ratio.
However, such a surface emission laser diode of the band shorter than 850 nm, there is caused a decrease in the band discontinuity between the quantum well active layer and the barrier layer or the spacer layer, and the efficiency of carrier confinement to the active layer is decreased. Thereby, there arises a problem in that it is difficult to attain good temperature characteristics as compared with the surface-emission laser diode of the 850 nm band. Further, because the active layer is added with active Al, there is a tendency that oxygen is incorporated into the active layer during the growth or processing thereof, while this leads to the problem of formation of non-optical recombination center, which in turn invites degradation of efficiency of optical emission and degradation of reliability.
Patent Reference 2 proposes a surface-emission laser diode (780 nm band) that uses an Al-free active region (quantum well active layer and the layers adjacent thereto) in a surface-emission laser diode of the wavelength band of 850 nm or shorter for the purpose of suppressing the formation of non-optical recombination center.
With this conventional surface-emission laser diode, GaAsP having a tensile strain is used for the quantum well active layer, and GaInP having a compressive strain is used for the barrier layer. Further, lattice matched GaInP is used for the spacer layer (the layer between the cladding layer and the first and third quantum well active layer), and AlGaInP is used for the cladding layer. According to the technology of Patent Reference 2, reliability of the surface-emission laser diode is improved in view of use of the Al-free composition of the active region.
Further, Non-Patent Reference 2 proposes a surface-emission laser diode of the 780 nm band that uses GaInPAs having a compressive strain for the quantum well active layer for the purpose of attaining the effect of Al-free active region and further for the purpose of increasing the gain of the active layer, wherein Non-Patent Reference 2 further teaches the use of lattice matched GaInP or GaInP having a tensile strain for the barrier layer and the use of lattice matched AlGaInP for the spacer layer (the layer between the cladding layer and the first and third quantum well active layers) and further the use of AlGaInP (having Al composition larger than the spacer layer) for the cladding layer.
With the surface-emission laser diode of Non-Patent Reference 2, the barrier layer has a lattice matched composition, and thus, the barrier layer has a larger bandgap as compared with the compressive strain composition, and thus, the efficiency of carrier confinement is improved as compared with the structure of Patent Reference 2 mentioned before.
With regard to polarization control, Patent Reference 3 shows the technology of using a substrate having a surface orientation inclined in the direction of (111)A surface or (111)B surface from the (100) surface orientation by the angle (inclination angle) of 15-40°, wherein this technology uses the anisotropy of optical gain in combination with the multiple quantum well active layer of InAlGaAs and InGaAsP having a compressive strain for increasing the optical gain in the inclined direction.
Further, Patent Reference 4 shows the method that forms the mesa shape in a circular, elliptical or elongated circular shape and sets the direction of the major axis in the direction of (111)A surface or (111)B surface from (100). In this case, a substrate having a surface orientation offset by 2° (including −5°-+5°) in the [110] direction from (100) is used. It should be noted that this is not the substrate inclined in the A surface or B surface direction.
However, it has not been realized a surface-emission laser diode of the wavelength shorter than 850 nm and at the same time having a large gain for the active layer and small threshold value, high output power, excellent reliability and controlled polarization direction.
Thus, while Patent Reference 2 can provide improved reliability in view of the use of Al-free active layer, the reference is silent about the control method of polarization. Further, while Non-Patent Reference 2 provides a structure of excellent carrier confinement, it is silent about the control method of polarization. Further, while Patent Reference 3 enables control of polarization direction, it is totally indifferent to reliability or structure matched to the property of the materials. Further, while Patent Reference 4 can control the polarization direction, it is totally indifferent about achieving high gain and long lifetime for the surface-emission laser diode of the wavelength shorter than 850 nm.
Further, in the case a material of (Al)GaInP system is used for the material forming the cavity region sandwiched by the upper and lower reflectors as described in Non-Patent Reference 2, it is known that there arises large increase of threshold current because of separation (segregation) of In such as carry-over of In into the AlGaAs layer at the interface between the cavity region and the upper reflector, which is formed by the material of the AlGaAs system.
Further, AlGaInP, a quaternary mixed crystal, has large thermal resistance, and there also arises a problem with a material of the (Al)GaInP system in that Zn (zinc), which is used for the p-type dopant, easily causes diffusion.
Thus, it has not been realized conventionally to provide a surface-emission laser diode of the wavelength shorter than 850 nm, having large gain for the active layer and low threshold value, high output, excellent reliability and controlled polarization direction.
In an aspect of this disclosure, there is provided a surface-emission laser diode having small Ga content or small change of Ga/Al ratio in a semiconductor distributed Bragg reflector (DBR) and having improved heat dissipation, and having a construction that can improve the controllability of etching at the time of forming the mesa structure by etching a laser stacking structure, and that is capable of performing high output operation.
In another aspect of this disclosure, there is provided a surface-emission laser diode of the wavelength shorter than 850 nm, the surface-emission laser diode having excellent reliability and being a high-output surface-emission laser diode of low threshold value while having a large gain of active layer.
In another aspect of this disclosure, there is provided a surface-emission laser diode of the wavelength shorter than 850 nm, the surface-emission laser diode having excellent reliability, large gain for the active layer and low threshold value, high output power and controlled polarization direction.
In another aspect, there is provided any one or more of a surface-emission laser diode array, an image forming apparatus, an optical pickup system, an optical transmission module, an optical transceiver module and an optical telecommunication system in which the foregoing surface-emission laser diode is integrated.
In another aspect of this disclosure, there is provided a surface-emission laser diode, comprising:
a semiconductor substrate;
a cavity region formed over said semiconductor substrate, said cavity region comprising: an active layer structural part including at least one quantum well active layer producing a laser light and a barrier layer; and a spacer layer provided in a vicinity of said active layer structural part, said spacer layer comprising at least one material; and
an upper reflector and a lower reflector provided over said semiconductor substrate respectively at a top part and a bottom part of said cavity region,
said cavity region, said upper reflector and said lower reflector forming a mesa structure over said semiconductor substrate,
said upper reflector and said lower reflector constituting a semiconductor distributed Bragg reflector having a periodic change of refractive index and reflecting an incident light by interference of optical waves,
at least a part of said semiconductor is distributed Bragg reflector being formed of a layer of small refractive index of AlxGa1-xAs (0<x≦1) and a layer of large refractive index of AlyGa1-yAs (0≦y<x≦1),
said lower reflector being formed of a first lower reflector having a low-refractive index layer of AlAs and a second lower reflector formed on said first lower reflector, said second lower reflector having a low-refractive index layer of AlGaAs,
wherein any one layer constituting said cavity region contains In.
In another aspect of this disclosure, there is provided a surface-emission laser diode, comprising:
a (100) GaAs substrate having a surface orientation inclined in a direction of a (111)A surface by an angle of 5° to 20°;
a cavity region provided over said GaAs substrate, said cavity region including an active layer structural part comprising at least one layer of quantum well active layer producing a laser light and barrier layers, and a spacer layer provided in a vicinity of said active layer structural part, said spacer layer comprising at lease one material; and
an upper reflector and a lower reflector is provided at a top part and a bottom part of said cavity region,
said cavity region and said upper and lower reflectors forming a mesa structure over said GaAs substrate,
said upper reflector and said lower reflector comprising a semiconductor distributed Bragg reflector having a periodic change of refractive index and reflecting an incident light by interference of optical waves,
at least a part of said semiconductor distributed Bragg reflector being formed of a layer of small refractive index of AlxGa1-xAs (0<x≦1) and a layer of large refractive index of AlyGa1-yAs (0≦y<x≦1),
a part of said spacer layer comprising (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1),
said quantum well active layer comprising GacIn1-cPdAs1-d (0≦c≦1, 0≦d≦1),
said barrier layers comprising GaeIn1-ePfAs1-f (0≦e≦1, 0≦f≦1),
said quantum well active layer having a compressive strain,
said active layer structural part having a shape anisotropy elongated in a direction of a (111)A surface as viewed from a direction of light emission.
In another aspect of this disclosure, there is provided a method of fabricating a surface emission laser diode, said surface emission layer diode comprising, over a semiconductor substrate: a cavity region comprising an active layer structural part including at least one quantum well active layer producing a laser light and barrier layers, and a spacer layer of at least one material provided in a vicinity of said active layer structural part; and an upper reflector and a lower reflector provided at a top part and a bottom part of said cavity region, said method comprising the steps of:
forming a stacked structure including said lower reflector, said cavity region and said upper reflector over said semiconductor substrate; and
forming a mesa structure by patterning said stacked film by dry etching,
said step of forming said stacked structure including a step of incorporating In to any one layer constituting said cavity region,
said step of forming said mesa structure by said dry etching comprises a step of controlling a height of said mesa structure by monitoring light emission of In.
In another aspect of this disclosure, there is provided a surface-emission laser diode, comprising:
a GaAs substrate;
a cavity region formed over said GaAs substrate, said cavity region including at least one quantum well active layer producing a laser light and barrier layers; and
an upper reflector and a lower reflector provided at a top part and a bottom part of said cavity region over said GaAs substrate,
said upper reflector and/or said lower reflector including a semiconductor Bragg reflector,
at least a part of said semiconductor distributed Bragg reflector comprising a semiconductor layer containing Al, Ga and As as major components,
wherein there is provided, between said active layer and said semiconductor layer that contains Al, Ga and As as major components, a semiconductor layer containing Al, In and P as major components such that said semiconductor layer containing Al, In and P as major components is provided adjacent to said semiconductor layer that contains Al, Ga and As as major components,
an interface between said semiconductor layer containing Al, Ga and As as major components and said semiconductor layer containing Al, In and P as major components being formed coincident to a location of a node of electric field strength distribution.
In another aspect of this disclosure, there is provided a surface-emission laser diode, comprising:
a GaAs substrate;
a cavity region formed over said GaAs substrate and having at least one quantum well active layer producing a laser light and barrier layers; and
an upper reflector and a lower reflector provided at a top part and a bottom part of said cavity region over said GaAs substrate,
said upper reflector and/or lower reflector including a semiconductor distributed Bragg reflector,
at least a part of said semiconductor distributed Bragg reflector comprising a semiconductor layer containing Al, Ga and As as major components,
there being provided, between said active layer and said semiconductor layer containing Al, Ga and As as major components, a (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) layer adjacent to said semiconductor layer containing Al, Ga and As as major components,
said (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) layer being added with Mg (magnesium) as a p-type dopant,
said semiconductor layer containing Al, Ga and As as major components being added with C (carbon) as a p-type dopant.
In another aspect of this disclosure, there is provided a surface-emission laser diode, comprising:
a GaAs substrate;
a cavity region formed over said GaAs substrate, said cavity region including at least one quantum well active layer producing a laser light and barrier layers; and
an upper reflector and a lower reflector provided at a top part and a bottom part of said cavity region over said GaAs substrate,
said upper reflector and/or lower reflector including a semiconductor distributed Bragg reflector,
at least a part of said semiconductor distributed Bragg reflector comprising a semiconductor layer containing Al, Ga and As as major components,
there being provided, between said active layer and said semiconductor layer containing Al, Ga and As as major components, a (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) layer adjacent to said semiconductor layer containing Al, Ga and As as major components,
said (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) layer being a semiconductor layer formed of a short period superlattice structure of AlInP and GaInP.
In another aspect of this disclosure, there is provided a surface-emission laser diode, comprising:
a GaAs substrate;
a cavity region formed over said GaAs substrate, said cavity region including at least one quantum well active layer producing a laser light and barrier layers; and
an upper reflector and a lower reflector provided at a top part and a bottom part of said cavity region over said GaAs substrate,
said upper reflector and/or lower reflector including a semiconductor distributed Bragg reflector,
at least a part of said semiconductor distributed Bragg reflector comprising a low refractive index layer of AlxGa1-xAs (0<x≦1) and a high refractive index layer of AlyGa1-yAs (0≦y<x≦1),
one of said low refractive index layers constituting said upper reflector and/or said lower is reflector and located closest to said active layer comprising (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1),
an interface between said cavity region and said low refractive index layer of said upper reflector and/or said lower reflector located closest to said active layer being coincident to an anti-node of an electric strength distribution.
When any one layer constituting the cavity region contains In, precision and reproducibility of mesa etching is improved by detecting exposure of said In-containing layer at the time of the mesa etching of the laser lamination structure comprising the cavity and the upper and lower semiconductor DBRs. Even in the case the lower semiconductor DBR contains AlAs/(Al)GaAs-DBR of excellent heat dissipation, it is possible to realize a construction in with the AlAs/(Al)GaAs-DBR is provided to the neighbor of the cavity. With such a construction, temperature rise at the time of laser driving is suppressed and a high output power surface-emission laser diode of excellent temperature characteristics is provided. At the same time, it becomes possible to provide a surface-emission laser diode having excellent uniformity in the laser characteristics and characterized by excellent reproducibility in the processing and excellent yield.
Particularly, by incorporating In into the upper or lower spacer layer constituting the active region and having much larger thickness as compared with the active layer structural part, it becomes possible to form the mesa structure with further improved reproducibility and further improved precision, and with this, it becomes possible to form a surface-emission laser diode of further improved temperature characteristics, higher output power and further improved uniformity in laser characteristics, with higher reproducibility of processing and with higher yield.
Further, by forming the semiconductor DBR constituting the second lower reflector in the aforementioned surface-emission laser diode, such that the semiconductor DBR has the thickness of 10 pairs or less, the thickness of the semiconductor DBR is set to be larger than the precision of the mesa etching and at the same time minimum. With this, temperature rise at the time of driving is suppressed further, and a surface-emission laser diode of high output power is obtained with excellent temperature characteristics.
Further, by forming a part of the spacer layer of the aforementioned surface-emission laser diode by (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) and by forming the quantum well active layer by GacIn1-cPdAs1-d (0≦c≦1, 0≦d≦1), and further by forming the barrier layer by GaeIn1-ePfAs1-f (0≦e≦1, 0≦f≦1), and further by using an AlGaInP material and thus (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) for a part of the spacer layer, it becomes possible to increase the bandgap difference between the spacer layer and the quantum well active layer as compared with the case of forming the spacer layer by the AlGaAs system. Thereby, there is achieved improvement of carrier confinement efficiency, and it becomes possible to realize a high-output laser having a further lower threshold value in combination with the excellent heat dissipation effect pertinent to such a structure.
Further, in the aforementioned surface emission laser diode, a GaInPAs material can be used for the barrier layer and the quantum well active layer, and thus, the quantum well active layer is formed of GacIn1-cPdAs1-d (0≦c≦1, 0≦d≦1), and that the barrier layers are formed of GaeIn1-ePfAs1-f (0≦e≦1, 0≦f≦1). Thus, the active layer structural part formed of the quantum well active layer and the layer adjacent thereto does not contain Al, and the problem of incorporation of oxygen into the active layer structural part by Al, and associated problem of formation of non-optical recombination center, can be suppressed, and a surface-emission laser diode of long lifetime can be realized.
Further, by forming the quantum well active layer with a compressive strain composition in the aforementioned surface-emission laser diode such that a compressive strain is accumulated in the quantum well active layer, the carrier confinement effect is augmented with the effect of the strain, and the optical gain of the active layer structural part is increased further. Further, improvement of heat dissipation is added to the foregoing. Thereby, the threshold value is decreased further. Thus, it becomes possible to realize a surface-emission laser diode of extremely high efficiency and high output power. Further, with decrease of the threshold value attained by the improvement of carrier confinement efficiency and further by the increase of gain as a result of use of the strained quantum well active layer, it becomes possible to decrease the reflectivity of the DBR at the exit side of light (upper semiconductor DBR). As a result of decrease of the reflectivity of the DBR at the optical exit side, a further increase of the optical power becomes possible.
Further, because the semiconductor substrate is a (100) GaAs substrate having a surface orientation inclined in the direction of a (111)A surface with an angle in the range from 5° to 20° (in other words, as a result of use of the (100) GaAs substrate having the surface orientation inclined in the direction of the (111)A surface with the angle of 5° to 20° by taking into consideration the surface orientation of the substrate) in the aforementioned surface-emission laser diode, adverse effects to the device characteristics of the semiconductor layer, such as decrease of bandgap caused by formation of natural super lattices or deterioration of surface morphology or formation of non-optical recombination centers caused by hillock (hill-like defect), are reduced.
Further, a surface-emission laser diode that uses the (100) GaAs substrate having the surface orientation inclined in the direction of the (111)A surface within the angular range of 5° to 20° (in other words the (100)GaAs substrate having the surface orientation inclined in the direction of the (111)A surface within the angular range of 5° to 20° with regard to the control of polarization) cannot attain the polarization control effect as in the case of using the (311)B substrate (corresponds to 25° inclination) currently drawing attention, and the anisotropy of optical gain attained with the use of inclined substrate becomes inevitably small. In such surface-emission laser diode, the foregoing decrease of anisotropy of optical gain can be compensated for by providing a compressive strain to the quantum well active layer and induce increase of anisotropy of the optical gain. Thereby, it becomes possible to control the polarization direction effectively. Thus, there is induced a synergistic effect of improvement of heat dissipation efficiency and increase of gain of the active layer structural part, and it becomes possible to realize a high output power surface-emission laser diode oscillating at a wavelength shorter than 850 nm and at the same time having low threshold value, excellent reliability and controlled polarization direction.
Next, with the use of the AlGaInP material, and thus (AlaGa1-a)In1-bP (0<a≦1, 0≦b≦1) for a part of the space layer, it becomes possible to secure a large bandgap between the spacer layer and the quantum well active layer as compared with the case of forming the spacer layer with the AlGaAs system, and the efficiency of carrier confinement is improved. Further, the threshold of laser oscillation is decreased and it becomes possible to increase the output.
In the aforementioned surface-emission laser diode, the active layer structural part formed of the quantum well layer and the layer adjacent thereto has an Al-free construction as a result of use of GaInPAs material for the barrier layers and the quantum well active layer, and as a result, incorporation of oxygen into the active layer structure by oxygen is reduced, and formation of the non-recombination centers is suppressed. With this, a surface-emission laser diode of long lifetime is realized.
Further, it becomes possible to decrease the threshold value of laser oscillation further with the effect of the compressive strain, by using a compressive strain composition for the quantum well active layer, and it becomes possible to increase the efficiency of laser oscillation and obtain large output.
Further, in the aforementioned surface-emission laser diode, there is caused further decrease of threshold value of laser oscillation because of the improvement of efficiency of carrier confinement to the active layer structural part and because of improvement of gain by the use of the strained quantum well active layer, and it becomes possible to decrease the reflectivity of the exit-side DBR. Thereby, it becomes possible to obtain further higher output.
Further, in the aforementioned surface-emission laser diode, it becomes possible, with the use of the (100)GaAs substrate having a surface orientation inclined in the direction of a (111)A surface with an angle within the range of 5° to 20° by taking into consideration the effect of the surface orientation of the substrate, to suppress the adversary the effects to the device characteristics of the laser such diode as decrease of bandgap caused by formation of natural super lattices, deterioration of surface morphology caused by hillocks (hill-like defect), formation of non-optical recombination centers, or the like.
With the polarization control, the surface-emission laser diode according to the second aspect of the present invention cannot utilize the effect attained by using the (311)B substrate, which is through most promising at the present juncture as noted before, and the anisotropy of optical gain associated with the use of inclined substrate becomes inevitably small. With the present invention, this decrease is compensated for, by increasing the anisotropy of optical gain attained by providing a compressive strain to the quantum well active layer, and by increasing the optical gain in the inclined direction of the substrate (111)A surface direction) by providing anisotropy in the outer shape of the active layer as viewed from the optical emission direction of the surface-emission laser diode. With this, control of polarization direction becomes extremely easy.
Thus, in the aforementioned surface-emission laser diode it becomes possible to realize a high output power surface-emission laser diode oscillating at the wavelength shorter than 850 nm and having a large optical gain for the active layer structural part, small threshold value of laser oscillation, excellent reliability, and controlled polarization plane.
Further, in the aforementioned surface-emission laser diode, it becomes possible to increase the band discontinuity to the quantum well active layer by accumulating a tensile strain in the barrier layers. Thereby, it becomes possible to increase the gain. With this, the threshold value of laser oscillation is decreased and the surface-emission laser diode becomes possible to perform high output power operation. With the material of the GaInPAs system, it should be noted that the semiconductor material constituting the barrier layer can increase the bandgap by decreasing the lattice constant.
Further, in the aforementioned surface-emission laser diode, it becomes possible to realize a surface-emission laser diode of the oscillation wavelength larger than about 680 nm. Further, as a result of the use of the AlGaInP system material for the spacer layer, it becomes possible to realize the carrier confinement equivalent to or superior to the case of the surface-emission laser diode of the 780 nm band that uses the active layer of the AlGaAs system, even in the case the active layer, formed of the quantum well layer and barrier layer, is formed of a material free from Al, as long as the compositional wavelength is 680 nm or longer. Further, the effect of the strained quantum well active layer is added thereto. Thus, it becomes possible to realize the characteristics equivalent to or superior to the surface-emission layer diode of the 780 nm band that has the active layer of the AlGaAs system.
Further, with the aforementioned method of fabricating a surface emission laser diode, said surface emission layer diode comprising, over a semiconductor substrate: a cavity region comprising an active layer structural part including at least one quantum well active layer producing a laser light and barrier layers, and a spacer layer of at least one material and provided in a vicinity of said active layer structural part; and an upper reflector and a lower reflector provided at a top part and a bottom part of said cavity region, said method comprising the steps of: forming a stacked structure including said lower reflector, said cavity region and said upper reflector over said semiconductor substrate; and forming a mesa structure by patterning said stacked film by dry etching, said step of forming said stacked structure including a step of incorporating In to any one layer constituting said cavity region, said step of forming said mesa structure by said dry etching comprises a step of controlling a height of said mesa structure by monitoring light emission of In, it becomes possible to detect the cavity part positively in the foregoing dry etching process, it becomes possible to form the mesa structure with good reproducibility and with excellent precision, and it becomes possible to fabricate such a surface-emission laser diode with good reproducibility and good yield.
By providing, in the aforementioned surface-emission laser diode, the interface between the semiconductor layer containing Al, Ga and As as major components and the semiconductor layer containing Al, In and P as major components to be coincident to a location of a node of electric field strength distribution, it becomes possible to decrease the effect of optical absorption at the foregoing interface significantly, even in the case there is caused some segregation of In at the time of crystal growth of the semiconductor layer containing Al, Ga and As as the major components on the semiconductor layer containing Al, In and P as the major components, and it becomes possible to suppress the adversary effect of increase of threshold value caused by the segregation of In.
By adding, in the aforementioned surface-emission laser diode, Mg (magnesium) to the semiconductor layer containing Al, In and P as the major components as a p-type dopant, and by adding C (carbon) to the semiconductor layer containing Al, Ga and As as the major components as a p-type dopant, it becomes possible to suppress the diffusion of dopant and reduce the memory effect, and it becomes possible to carry out the doping with good controllability. Thereby, a doping profile near the designed profile is obtained, and degradation of the crystal quality of the active layer is suppressed. With this, a high output power surface-emission laser diode having a low threshold value can be realized easily.
Further, with the aforementioned surface-emission laser diode, the efficiency of heat dissipation is improved and high output operation is realized easily by pseudo-constructing the AlGaInP mixed crystal by AlInP having small thermal resistance and GaInP.
Further, it becomes possible, with the user of the spacer layer of (AlaGa1-a)bIn1-bP (0<a≦1, 0≦b≦1) in the surface-emission laser diode, to increase the bandgap difference between the spacer layer and the quantum well active layer as compared with the case of forming the spacer layer by the AlGaAs system. Thereby, the threshold of laser oscillation is decreased, the efficiency of laser oscillation is improved, and high output operation is realized. Further, by using GacIn1-cPdAs1-d (0≦c≦1, 0≦d≦1) for the quantum well active layer and by GaeIn1-ePfAs1-f (0≦e≦1, 0≦f≦1) for the barrier layers, it becomes possible to construct the active layer by a material free from Al, and the active region formed of the quantum well active layer and the adjacent layer becomes Al-free. Thereby, it becomes possible to reduce the incorporation of oxygen, and formation of the non-optical recombination centers is suppressed. Thereby, it becomes possible to realize a surface-emission laser diode of long lifetime. Thus, a high output power surface-emission laser diode of the wavelength of 850 nm or shorter and having a large gain for the active layer, low threshold value of laser oscillation and good reliability is realized.
Further, with the aforementioned surface-emission laser diode, it is possible to reduce the threshold value of laser oscillation by the effect of strain, by using the compressive strain composition for the quantum well active layer, and the efficiency of laser oscillation is improved. Further, as a result of improvement of the carrier confinement efficiency and increase of the gain attained by the use of the strained quantum well active layer, the threshold of laser oscillation is decreased further, and it becomes possible to reduce the reflectivity of the exit-side DBR. With this, it becomes possible to increase the laser output further.
Further, it becomes possible to increase the degree of freedom of design such as use of the quantum well active layer of larger strain, by compensating for the strain of the quantum well active layer in the surface-emission laser diode. Further, because the material of the smaller lattice constant has a larger bandgap in the semiconductor material of the GaInPAs system and used for the barrier layer, it becomes possible to increase the band discontinuity to the quantum well active layer. Thereby, the gain is increased and it becomes possible to carry out low-threshold value operation and high output power operation.
Further, with the aforementioned surface-emission laser diode, it becomes possible, by constructing the lower reflector such that the lower reflector includes AlAs having small thermal resistance for the low refractive index layers, the dissipation characteristics of heat generated in the active layer are improved, and the temperature rise at the time of driving is suppressed. Thus, a high output power surface-emission laser diode of excellent temperature characteristics is obtained.
Further, with the aforementioned surface-emission laser diode, junction of the AlGaInP system material and the AlGaAs system material is made easily by interposing an intermediate layer of small Al content between the low refractive index layer and the high refractive index layer of the semiconductor distributed Bragg reflector. Thus, in the case of laminating the AlyGal-yAs (0≦y, x≦1) high refractive index layer on the (AlaGa1-a)In1-bP (0<a≦1, 0≦b≦1) low refractive index layer, it becomes possible to conduct the growth of the high refractive index layer in a wide conditional range by reducing the Al content at the interface. Further, with such a construction, the band discontinuity of the valence band is decreased and the resistance against the current flowing in the stacked direction is reduced.
Further, with the aforementioned surface-emission laser diode, it becomes possible, as a result of the use of the AlGaInP system spacer layer, it becomes possible to realize the carrier confinement equivalent to or superior to the case of the surface-emission laser diode of the 780 nm band that uses the active layer of the AlGaAs system, even in the case of using the Al-free active layer (quantum well layer and barrier layer), as long as the compositional wavelength is 680 nm or longer.
Further, with the aforementioned surface-emission laser diode, it becomes possible to reduce the adversary effects to the device characteristics of the semiconductor layer, such as decrease of bandgap caused by formation of natural super lattices or deterioration of surface morphology or formation of non-optical recombination centers caused by hillock (hill-like defect), by using the (100) GaAs substrate having a surface orientation inclined in the direction of a (111)A surface with an angle in the range from 5° to 20°. Further, it becomes possible to conduct polarization control by utilizing the nature of the anisotropic substrate. When it is not possible to attain the polarization control as in the case of using the (311)B substrate, which is currently thought most promising, and the anisotropy of optical gain attained with the use of inclined substrate becomes inevitably small, the surface-emission laser diode can successfully compensate for the foregoing decrease of anisotropy of optical gain by providing a compressive strain to the quantum well active layer and induce increase of anisotropy of the optical gain. Thereby, it becomes possible to improve the controllability of the polarization direction with such a surface-emission laser diode.
Further, with the aforementioned surface-emission laser diode, it becomes possible to improve the controllability of the polarization direction, by providing anisotropy to the peripheral shape of the active layer as viewed from the optical exit direction of the surface-emission laser diode such that there is formed a shape elongated in the (111)A direction, such that the effect of increase of the optical gain in the inclined direction of the substrate ((111)A surface direction) is added.
Further, by forming the surface-emission layer diode on the same substrate in plural numbers, it is possible to construct a surface-emission laser diode array. Thus, precision and controllability of mesa formation are improved, and it becomes possible to produce the surface-emission laser diode array having uniform laser characteristics and good processing reproducibility, with high yield and low cost.
Particularly, with the use of the aforementioned surface-emission laser diode, thermal interference between the elements in the array is suppressed because of the improvement of the heat dissipation characteristics, and it becomes possible to form a high-density array in which the surface-emission laser diode elements are disposed with closer distance from each other.
Further, by applying the construction of integrating a large number of surface-emission laser diodes that are, capable of performing high output power operation, on the same substrate for the image writing optical system of an electron photographic image forming apparatus, it becomes possible to achieve high-speed writing by using plural beams at the same time, and the writing speed is improved significantly. Thereby, it becomes possible to carry out printing without decreasing the speed even in the case the density of the writing dots is increased. Further, when compared at the same dot density, the image forming apparatus that uses such a surface-emission laser diode enables printing at higher speed as compared with the conventional image forming apparatuses. Further, when the aforementioned surface-emission laser diode is applied to communication, the data transmission is made by a large number of beams simultaneously, and high-speed communication is realized.
Further, the surface-emission laser diode operates at low power consumption, and thus, it becomes possible, when operated in the state of being incorporated into an apparatus, to reduce the temperature rise in the apparatus.
Further, by using the high output power surface-emission laser diode or the surface-emission laser diode array of such high output power surface-emission laser diodes for the writing optical source, it becomes possible to improve the printing speed as compared with the image forming apparatus that uses a conventional surface-emission laser diode.
Alternatively, in the case of printing at the conventional speed, it becomes possible to reduce the number of the laser arrays, and the yield of production of the surface-emission laser diode array chip is improved significantly. Further, it becomes possible to reduce the cost of the image forming apparatus. In the case the surface-emission laser diode capable of controlling the polarization plane is used, reliability of image formation is improved. Further, in the case a surface-emission laser diode free from Al in the active layer structural part formed of a quantum well active layer and a spacer layer is used, lifetime comparable to the surface-emission laser diode for telecommunication purposes such as the surface-emission laser diode of the 850 nm band is attained, and thus, it becomes possible to reuse the optical writing optical unit. Thereby, the load to the environment is reduced.
Further, by using the aforementioned surface-emission laser diode or the surface-emission laser diode array that uses such surface-emission laser diodes for the optical source of optical pickup, it becomes possible to realize a handy type optical pickup system of long battery life. In conventional compact disk devices, a semiconductor layer of 780 nm wavelength is used for the optical writing and playback of recording medium, wherein a surface-emission laser diode has power consumption smaller than that of an edge-emission laser diode by a factor of 1/10.
Further, by using the aforementioned surface-emission laser diode or the surface-emission laser diode array of such surface-emission laser diodes for the high-power optical source of optical transmission module or optical transceiver module, it becomes possible to construct an economical high-speed optical transmission system that uses a low cost POF (plastic optical fiber).
With the optical transmission that uses an acrylic POF, a surface-emission laser diode of the oscillation wavelength of 650 nm has been used conventionally for the optical source in view of the absorption loss characteristics of the optical fiber, while the use of the surface-emission laser diode in practical purposes is not been successful. Because of this, LEDs are used currently, while an LED is difficult to perform high-speed modulation, and it is indispensable to provide a laser diode in order to realize high-speed transmission exceeding 1 Gbps.
With the aforementioned surface-emission laser diode having the wavelength of 680 nm or longer, a large gain is attained for the active layer and it is possible to provide a large output. Further, the surface-emission laser diode has excellent high temperature characteristics. Thus, by using such a surface-emission laser diode, it is possible to achieve, in spite of the fact there is an increase of absorption loss by the fiber, optical transmission of short range. Thus, an economical high-speed optical transmission module or an optical transceiver module that combines a low cost POF with a low cost optical source of surface-emission laser diode is realized. Further, an optical communication system that uses these is realized. Because such an optical communicating system is extremely economical, it is suited for the optical communication systems of home use, or for use in office rooms, or for the use inside an apparatus.